1. Field of the Invention
The present invention relates to meter electronics and methods for determining a mass fraction of flow components in a flow material flowing through a flow meter.
2. Statement of the Problem
It is known to use Coriolis mass flow meters to measure mass flow and other information of materials flowing through a pipeline as disclosed in U.S. Pat. No. 4,491,025 issued to J. E. Smith, et al. of Jan. 1, 1985 and Re. 31,450 to J. E. Smith of Feb. 11, 1982. These flow meters have one or more flow tubes of different configurations. Each conduit configuration may be viewed as having a set of natural vibration modes including, for example, simple bending, torsional, radial and coupled modes. In a typical Coriolis mass flow measurement application, a conduit configuration is excited in one or more vibration modes as a material flows through the conduit, and motion of the conduit is measured at points spaced along the conduit.
The vibrational modes of the material filled systems are defined in part by the combined mass of the flow tubes and the material within the flow tubes. Material flows into the flow meter from a connected pipeline on the inlet side of the flow meter. The material is then directed through the flow tube or flow tubes and exits the flow meter to a pipeline connected on the outlet side.
A driver applies a force to the flow tube. The force causes the flow tube to oscillate. When there is no material flowing through the flow meter, all points along a flow tube oscillate with an identical phase. As a material begins to flow through the flow tube, Coriolis accelerations cause each point along the flow tube to have a different phase with respect to other points along the flow tube. The phase on the inlet side of the flow tube lags the driver, while the phase on the outlet side leads the driver. Sensors are placed at different points on the flow tube to produce sinusoidal signals representative of the motion of the flow tube at the different points. The phase difference between the two sensor signals is proportional to the mass flow rate of the material flowing through the flow tube or flow tubes.
One application of a vibrating flow tube device as described above is in measuring a mass flow rate of a flow material. However, in some flow measurement environments, the flow material comprises a multi-phase flow that includes two or more of a fluid phase, a gas phase, and a solid phase. A common multi-phase flow material comprises a fluid flow material that includes entrained gas, such as air, for example.
A prior art flowmeter cannot accurately, quickly, or satisfactorily track or determine a pickoff sensor frequency during a two-phase flow of the flow material. Prior art vibratory flowmeters are designed to measure mass flow rate of a relatively stable and uniform flow material. However, because the flow measurement reflects the mass of the flow material, abrupt changes in mass can cause erroneous measurements or the mass flow changes are not even tracked by the flowmeter. For example, where the flow material includes entrained air, the air bubbles transiting the flowmeter can cause spikes in the frequency response of the flowmeter. These frequency errors can cause difficulty in determining an accurate mass flow rate and can be propagated through any subsequent calculations of other flow characteristics. Consequently, the phase determination is likewise slow and error prone, as the prior art derives the phase difference using the determined pickoff frequency. Therefore, any error in the frequency determination is compounded in the phase determination. The result is increased error in the frequency determination and in the phase determination, leading to increased error in determining the mass flow rate. In addition, because the determined frequency value is used to determine a mass flow rate and a density value (density is approximately equal to one over frequency squared), an error in the frequency determination is repeated or compounded in the mass flow and density determinations.
A prior art approach to metering a flow material does not satisfactorily measure individual components of a multi-phase flow. The prior art frequency determination is relatively slow. The prior art frequency determination typically characterizes the flow over a time period of at least 1-2 seconds and therefore produces an average frequency measurement value. The prior art approach is satisfactory for single phase flows and flows that change only slowly and modestly. Sharp changes are not measurable in the prior art. Accurate measurement of individual flow components cannot be achieved by the prior art. The prior art cannot accurately determine the mass of a multi-phase flow at a point in time. The prior art cannot determine the mass fraction of individual flow components of a multi-phase flow.